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Friday, July 15, 2005

Near-field Recording: Beyond Blu-Ray

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Near-field Recording: Beyond Blu-Ray

"Philips researchers have made significant progress in developing a new storage technology called near-field optical recording. The technology boosts capacity to 150 gigabytes stored on two layers of an optical media, not very different from today's DVDs.

The Near-field optical recording terminology refers to the extremely short distance between the read/write head and the disc surface. The roughly 25-nm gap is directly comparable to the distance between the head and the disk surface in hard-disk assemblies.

As in the case of Blu-Ray, the fundumental for achieving large storage capacities is to increase the data density recorded on the medium. But the data density of an optical recording medium depends on the focused laser beam spot size, which is limited by diffraction. The beam spot size can be reduced by using a shorter wavelength laser or a larger Numerical Aperture (NA) objective lens. In the case of CD media, we are talking about a 780nm laser and 0.45 NA (0.7GB), for DVD we have 650nm and 0.6 NA (4.7GB) and for the Blu-Ray there will be a "blue" 405nm laser, 0.85 NA (25GB).

Philips' approach for high density recording under the near field recording concept uses a 405 nm laser beam, focused by a pair of special lenses that offer a NA of 1.5-20! Through this, the capacity of a DVD medium can reach 150GB or more, on two layers.

The challenge for Philips engineers was the development of a high NA lens system combined with a low-hovering recording head.

The answer is to use a blue laser to write and read data through a "solid immersion lens" (SIL). This type of optics is already used in microscopes and in lithography equipment for semiconductor production. The SIL uses the different refractive index of glass and air to achieve a high numerical aperture. The SIL optical head is composed of a hemisphere which is made from high refractive index glass and high NA focusing objective lens."

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